1. Field of the Invention
This invention relates in general to a pressure head for bonding parts with heat and pressure and more specifically to a pressure head having a gimbaled horn that pivots on its push rod and having an adjustable zero-pressure position.
2. Description of the Related Art
A thrust head or head is commonly used to attach one item to another. The head must move relative to the workpiece. In some applications the workpiece is moved into contact with the head. In other applications, the head is moved by a thruster, such as a pneumatic cylinder. The horn, that is the working element of the head, may be heated for bonding work pieces together, that is one article to another with heat and pressure. For example, a ribbon cable circuit may be bonded to an electronic component or a flip chip may be bonded to the electronic package circuitry.
In many applications, but particularly when heat and pressure are to be applied in the bonding, it is imperative that the horn face, that is the pressure surface of the horn, align perfectly with the surface of the work pieces. One method of horn alignment to solve this problem has been to gimbal the horn such that, upon encountering the workpiece, the horn rotates to alignment.
In many applications with work pieces requiring very close bonding tolerances, it imperative that the force exerted on the horn by the workpiece required to align the gimbaled horn with the workpiece be extremely small. This is necessitated because the gimbaled horn not only rotates as it is aligned but also moves sideways thereby producing shear forces on the workpiece. If the rotational force is more than de minimis, then the shear force created in the rotation will disrupt the workpiece. Consequently, the mass of the gimbaled head needs to be minimized and the lever arm from the point of alignment force to the gimbal point or axis need to be maximized. Even the forces required to overcome the static friction of bearings in many conventional gimbaled horns or to overcome the moment and resistance to rotation created by wires leading to the heating element and thermocouple on the gimbaled horn are too large for use in some high tolerance applications.
Additionally, if the amount of necessary alignment rotation or average alignment rotation is known beforehand, it would be advantageous to pre-align, that is "zero", the horn beforehand as this would diminish or eliminate shear forces on the workpieces.
Therefore, there has been a need for a thrust head including apparatus for zero adjusting the gimbaled horn.
It is further desirable that the gimbaled horn is not fixed in the head in the up/down direction, such that, upon first encountering the workpiece the horn can move within the head such that much lower shear forces are generated on the workpiece.
It is further desirable that the gimbaled horn requires minimal force to align the horn face with the workpiece.